Three US sailors were killed when 21-year-old Mohammed Alshamrani, a member of the Royal Saudi Air Force who was training at Naval Air Station Pensacola, entered a building on base and "proceeded to walk around shooting down his unarmed victims in cold blood," Barr said.

Alshamrani, who was killed by law enforcement during the attack, had a history of airing his anti-American, anti-Israel and jihadi messages views on social media, including in a post on September 11 stating that "the countdown has begun," and another post made two hours before the attack, Barr said.

During a 15-minute shooting spree, Alshamrani shot at a photo of President Donald Trump as well as a former president, according to FBI Deputy Director David Bowdich. He also made statements while he was shooting that were critical of American servicemen overseas, Bowdich said.

The Pensacola shooting drew immediate scrutiny to the system of accepting foreign military trainees on US bases. Barr defended the program on Monday, calling Saudi Arabia "an important military partner," and said that Saudi Arabia had been fully cooperative in the investigation.

No other co-conspirators have been charged in the shooting, and Barr said Monday that investigators did not find evidence that any of the shooter's friends or fellow trainees from Saudi Arabia had advanced knowledge that he was going to attack the base.

Federal prosecutors evaluated each case and determined that none merited charges, Barr said, although the government of Saudi Arabia determined the material amounted to "conduct unbecoming" of a military officer, and the 21 trainees were disenrolled and returned on a flight to the kingdom Monday.

Investigators interviewed more than 500 people in the wake of the shooting and amassed dozens of terabytes of data.

Barr expressed frustration, however, that the FBI was still unable to access data stored on two iPhones belonging to the shooter because of Apple's built-in encryption.

Alshamrani stopped his shooting spree last month long enough to place one of his iPhones on the floor and shoot a bullet into it, Barr said Monday. The shattered device's screen was shown enlarged on posters beside the attorney general.

While experts at the FBI crime lab were able to fix the damaged phone, as well as another phone that the shooter had left in his car, investigators have been unable to get past their passwords despite court-authorized search warrants, Barr said.

"Both phones are engineered to make it virtually impossible to unlock them without the password. It is very important to know with whom and about what the shooter was communicating before he died," Barr said.

The FBI sent Apple a letter asking for its help in unlocking the phones last week, CNN has reported.

Barr would not say Monday whether the Justice Department planned to take Apple to court over access to the phones -- as they did in 2016 in a similar case involving a phone owned by the San Bernardino shooter. Barr said that the FBI had exhausted all other options to try and access the phone, including the use of third party companies.

Apple said in a statement last week that they had already provided investigators with all the information in their possession, but Barr said on Monday that the technology company had in fact not provided any substantive assistance. A spokeswoman for Barr later clarified that the attorney general was referring to assistance from Apple regarding access to data stored on the phone, as opposed to information that may have been saved in the cloud.

Apple has responded to the FBI's letter, although the company has yet to say if they have the ability themselves to get into the phone, according to Kerri Kupec, the DOJ spokeswoman.

"We call on Apple and other technology companies to help us find a solution so that we can better protect the lives of Americans and prevent future attacks," Barr said.

In a statement released on Monday night, Apple said the company rejects "the characterization that Apple has not provided substantive assistance" to the FBI in the Pensacola investigation. The company said in the statement that the bureau did not notify the company of a second iPhone until January 6, more than a month after the shooting. The company received a subpoena for the second phone on Wednesday and has responded to that subpoena, the statement read.

"We responded to each request promptly, often within hours, sharing information with FBI offices in Jacksonville, Pensacola and New York," the statement said. "The queries resulted in many gigabytes of information that we turned over to investigators. In every instance, we responded with all of the information that we had."

The company added, "We have always maintained there is no such thing as a backdoor just for the good guys. Backdoors can also be exploited by those who threaten our national security and the data security of our customers. Today, law enforcement has access to more data than ever before in history, so Americans do not have to choose between weakening encryption and solving investigations. We feel strongly encryption is vital to protecting our country and our users' data."

This story has been updated with additional information from Monday's news conference.